The objectives of the proposal are to determine the effects of silica dust inhalation on immunological defense mechanisms which are involved in pulmonary resistance to infection and some of the early structural changes occurring in lung cells. The specific effects of silica that are being studied include: the functions of T cells such as antigen recognition, cell lysis, and production of macrophage inhibitory factor; the lung macrophage associated activities; the ability of silica-dust exposed animals to respond to B cell dependent and B and T cell dependent antigens; the susceptibility of silica exposed animals to infection induced by an influenza virus aerosol; histologic determinations of changes in the cells in a variety of organs after silica inhalation. The most significant change observed in this continuing study is the suppression of antibody formation after silica inhalation. This suppression occurs in both mediastinal lymph nodes and spleens and is found whether the antigen is introduced as an aerosol or given by injections.